Semiconductor materials have many applications in modern technology. In particular, semiconductor materials are useful in the production of microelectronic components such as transistors and diodes. Whilst inorganic semiconductors such as elemental silicon have traditionally been employed in the production of these semiconductor devices, recently other materials having semiconducting properties have become available and are being adopted in the microelectronic industry.
A particularly noteworthy class of non-silicon semiconductors is that of organic semiconductors. Several families of organic compounds are known which exhibit semiconductor properties. One advantage of these organic semiconductors is that they can be subjected to solution processing in contrast to traditional inorganic materials such as silicon.
One specific problem which is encountered when using organic semiconductors is that these compounds tend to spontaneously become doped in the presence of air due to absorption of oxygen molecules. This spontaneous doping increases the conductivity of the organic semiconductor but this is not so desirable as it decreases the on/off ratio of the semiconductor. The on/off ratio is the ratio of the conductivity of a given semiconductor incorporated in a transistor when a high voltage is applied through the gate electrode of the transistor (the “on” state) to the conductivity when no voltage is applied (bulk conductivity) (the “off” state). The on/off ratio should be as high as possible, and values of greater than 500 are preferred. Therefore, it is important to minimise the bulk conductivity of the material.
In practice, where an organic semiconductor is doped with oxygen, its bulk conductivity is high, so that the increase in conductivity as the transistor is switched on is only very small. Therefore, the observed on/off ratio for doped organic semiconductors is relatively low. In contrast, a high on/off ratio is observed in the same organic semiconductor after removal of the dopant because the bulk conductivity is low.
In view of the importance of maximizing the on/off ratio of a semiconductor, it is common practice to dedope organic semiconductors at some stage during their processing and then seal the dedoped material to protect it from subsequent doping by air.
One known way of dedoping an organic semiconductor is to expose it to liquid hydrazine [P. Coppo et al., Macromolecules 36, 2705, 2003]. However, the use of hydrazine in a factory production line is highly undesirable in part because hydrazine is carcinogenic and in part because it is highly flammable and explosive, it being a well known rocket fuel.
As an alternative to chemical treatments, physical treatment such as heating in an inert atmosphere (e.g. in a nitrogen atmosphere or in a vacuum) are known [Z. Bao et al., Appl. Phys. Letts., 69 26, 1996 & D. B. A. Rep et al., Organic Electronics 4, 201, 2003]. These treatments only increase the on/off ratio by a factor of approximately 10 and so are also unsatisfactory.
Furthermore, it is known to dedope organic semiconductors electrochemically by potential-step chronocoulometry [Y. Kunugi et al., J. Mater. Chem. 10, 2673, 2003]. It is impractical however to carry out this method on an industrial scale.
Other than the dedoping methods described above, alternative dedoping methods exist which are expected to result in some degree of dedoping. Amongst these, it has been suggested to add metal particles in the form of a fine powder to a doped organic semiconductor or to add nanoparticles or nanotubes such as for example titania nanotubes to the semiconductor to be treated. However, these methods are generally not considered to be particularly useful because they result in the deposition of the dedopant material in the organic semiconductor material which is undesirable as this affects the physical properties of the semiconductor and consequently has an impact on the performance of a device produced from the material.
A further problem associated with the known methods of dedoping is that they all have to be performed on the organic semiconductor in its bulk form, that is before the semiconductor is fashioned into a semiconductor device. This is inherently associated with disadvantages because, once the organic semiconductor has been dedoped, it must be ensured that it is not exposed to air as this would result in re-doping by oxygen. Therefore, it has until now been common practice to assemble microelectronic devices comprising organic semiconductors in an inert atmosphere in order to avoid re-doping of the organic semiconductor and then to seal the assembled organic semiconductor devices before removing them from the inert atmosphere. This puts severe restrictions on the production line set-up for producing such semiconductor devices. There is therefore a need for a more cost-effective way of producing semiconductor devices based on organic polymers which avoids the need to maintain an inert atmosphere during production. In view of the various deficiencies of the prior art dedoping methods, there has been a need for the development of an improved method of dedoping organic semiconductors. Accordingly, the present inventors have sought to develop a new dedoping method which does not suffer from the deficiencies of the prior art methods discussed above.